The invention relates to a luminaire for projecting an image by means of a light beam, comprising
a housing having a light emission window, which housing accommodates a light source and an optical system for directing the radiation to be generated by the light source along a longitudinal axis, and
an image-forming surface which is to be placed in the light beam transversely to the longitudinal axis.
Such luminaires are known per se. They are used in, inter alia, theater lighting, such as stage lighting, in particular for projecting an image or a silhouette on a stage, screen, background or backcloth. Other applications of these luminaires include the projection of a logo, text or otherwise an image onto a facade of a building or on a road surface, for example in front of a shop window, for example, to attract the attention of a potential customer. Dynamic effects are generally brought about by providing the housing with moving mirrors. In addition, said luminaires generally comprise a plurality of image-forming surfaces, also referred to as GOBOs. In an alternative embodiment, said GOBOs are mounted on rotatable, transparent sheets, so that each time a different GOBO is imaged by the luminaire.
From EP-A 0 253 081, a so-called light pattern generator is known, which is used as theater lighting, wherein a halogen lamp or a metal halide lamp is used as the light source. Using a reflector, the light originating from the light source is turned into a parallel light beam which is focused on an image-forming surface by means of a lens. Subsequently, the image-forming surface is formed into a corresponding image.
Luminaires of the type mentioned above exhibit an important drawback. They often have large dimensions, are comparatively heavy and relatively expensive. In addition, such luminaires require comparatively much maintenance, for example, in that the lamp has to be regularly replaced.
It is an object of the invention to obviate the above-mentioned drawback. The invention more particularly aims at providing a luminaire of the type mentioned in the opening paragraph, wherein a simpler and cheaper construction is obtained in combination with a comparatively low energy consumption and relatively low maintenance costs.
To achieve this, the luminaire is characterized in accordance with the invention in that the light source comprises at least one light-emitting diode, and the optical system includes a collimating element for collimating the light beam.
Light-emitting diodes (LEDs) have a service life in excess of 60,000 hours, which is much longer than that of the halogen lamp or metal halide lamp used in the known luminaire. In addition, light-emitting diodes have a relatively low energy consumption (approximately 1 Watt instead of 50-200 Watt).
A luminaire having a high optical efficiency is obtained by using opto-electronic elements, also referred to as electro-optic elements, for example electroluminescent elements, such as light-emitting diodes (LEDs) as the light source. As a consequence of this and other factors, the decrease in light output by the use of light-emitting diodes is not proportional to the decrease of the power and the dimensions of the luminaire.
If a plurality of light-emitting diodes are used, they are preferably arranged in an array, with one collimating element being associated with each LED. In an alternative embodiment, the luminaire comprises a plurality of light-emitting diodes associated with a plurality of collimating elements, enabling various images of different colors to be alternately or simultaneously projected. One or more LEDs may be associated with one collimating element. As a result, the use of LEDs has the further advantage that dynamic lighting possibilities are created. If different types of LEDs are combined and/or LEDs of different colors are used, colors can be mixed in a desirable manner and also changes in color can be effected without having to use, for example, a so-called color wheel. To achieve this, a favorable embodiment of the luminaire in accordance with the invention comprises control electronics for changing the luminous flux of the LEDs. This measure enables the desired color effects to be achieved. A suitable combination of LEDs also enables white light to be obtained, and control electronics can be used to set a desired color temperature, which color temperature remains constant during operation of the luminaire. The use of this measure further enables the luminous flux to be dimmed. It is to be noted that the control electronics is customarily accommodated in the housing. In principle, however, the control electronics may be provided outside the housing.
A further drawback of the known luminaire is that, during operation of the luminaire, the light source used emits comparatively much heat, in practice, in the form of IR radiation. In addition, such a light source generates comparatively much UV-radiation. As a result, it proved impossible to use synthetic resin for the optical parts in the luminaire. It has been found that parts of the optical system which are made of synthetic resin are subject to degradation under the influence of the UV-radiation and/or they melt under the influence of the IR-radiation. Therefore, the optical system in the known luminaire is made of glass. Since LEDs generate much less radiation heat and/or UV light than gas discharge lamps or halogen lamps, it is possible in the luminaire in accordance with the invention to use synthetic resin for the parts of the optical system used to direct the radiation generated by the light source. The temperature of the lighting-emitting diode is much lower than that of the lamp in the known luminaire (approximately 50xc2x0 C. as compared to approximately 200xc2x0 C.). Furthermore, it is possible to adapt the optical path in the housing of the luminaire to the light source. In addition, optical parts of synthetic resin having the dimensional accuracy necessary for this application can be manufactured relatively readily and inexpensively as compared to glass parts. In this respect, satisfactory results are achieved using optical systems of poly[methylmethacrylate] (PMMA) or of polycarbonate (PC).
By using a collimating element, which surrounds the light-emitting diode, the use of a reflector as in the known luminaire is precluded. The collimating element further causes the light emitted, in operation, by the light-emitting diode to become a parallel light beam. In the collimating element, preferably, both reflection and total internal reflection of light take place, the provision of a reflective coating, as on the reflector in the known luminaire, being precluded.
The luminaire in accordance with the invention is compact, lightweight and has a low energy consumption and low maintenance costs.
An embodiment of the luminaire is characterized in accordance with the invention in that the light source is composed of a single light-emitting diode, the luminous flux of which is at least 5 lm during operation.
An advantage of the use of a single LED is that a very compact luminaire is obtained. At present, so-called high-efficiency, high-output LEDs are commercially available. The luminous flux per LED is at least 10 lm, preferably more than 30 lm per LED. The use of these high-power LEDs enables a luminaire having a very compact housing to be manufactured.
A favorable preferred embodiment of the invented luminaire is characterized in that the LED is mounted on a metal-core printed circuit board. If the LEDs are provided on such a metal-core printed circuit board (MC-PCB), heat generated by the LED or the LEDs can be readily dissipated via the PCB through heat conduction. In the case of a luminaire comprising a light source having only a single LED, the use of a MC-PCB is not necessary.
An interesting embodiment of the luminaire is characterized in accordance with the invention in that the housing is made of metal and provided with cooling fins, and in that the metal-core printed circuit board is in contact with the metal housing via a heat-conducting connection. Such a heat-conducting connection is preferably formed by mounting the MC-PCB on a metal plate which is connected to the metal housing. In this embodiment, the heat generated in the LED or LEDs can be dissipated, by (thermal) conduction, via the MC-PCB and the metal plate to the housing and the cooling fins, resulting in heat being dissipated to the environment. An advantage hereof is that, unlike the known luminaire, forced air cooling to dissipate heat is not necessary. Unlike the light source in the known lamp, where a halogen lamp or a metal halide lamp is used, an LED used as a light source does not emit heat. The heat originating from an LED is dissipated via conduction.
In a preferred embodiment of the luminaire, the optical system further comprises a focusing lens to focus the collimated light beam on the image-forming surface. In a preferred embodiment of the luminaire, the collimating element and the focusing lens form an integrated whole. This results in a greater compactness of the luminaire. In an alternative embodiment of the luminaire, the focusing lens is embodied so as to be a Fresnel lens. The focusing lens preferably consists of a synthetic resin, such as PMMA or PC, the desired optical Fresnel structure being obtained by means of injection molding.
In a preferred embodiment, the luminaire also comprises a projection lens for projecting an image of the image-forming surface. Preferably, the projection lens is embodied so as to be a Fresnel lens. This leads to a more compact luminaire. Such a Fresnel lens is preferably made of a synthetic resin, for example PMMA or PC. To avoid lens errors, the projection lens is preferably embodied so as to be an aspherical lens. In an alternative embodiment, the projection lens comprises diffractive structures, which lead to a higher image quality (correction of lens errors).
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.